(1) Field of the Invention
This invention relates to gas flow controllers for use with aircraft molecular sieve type gas separation systems and is more particularly concerned with a gas flow controller which is responsive to internal and external pressures so as to control the mass of air flowing through a molecular sieve type oxygen enrichment of air system supplying oxygen enriched air for breathing by aircrew.
(2) Description of the Prior Art
It is known to use a molecular sieve type gas separation system in an aircraft application to provide oxygen enriched air as breathable gas for aircrew. In this aircraft application air is bled from a compressor stage of a gas turbine engine used to power the aircraft, and supplied to the molecular sieve beds of the system by way of a pressure regulator and a heat exchanger. The sieve beds are usually operated in an overlapping sequence of fixed time cycles which comprise a charge/adsorption on-stream phase followed by a purge/desorption regeneration phase. In the charge/adsorption phase nitrogen is adsorbed by sieve material in the bed and oxygen enriched air is delivered as product gas. In the purge/desorption phase a small portion of product gas from an on-stream bed is fed as a backflow through the bed that is in the purge/desorption phase so that nitrogen is desorbed and flushed from the sieve material and to place the bed into a cleansed condition preparatory to its next charge/adsorption phase.
Such systems were originally treated as a source of substantially pure oxygen to be utilised in a manner traditional in aircrew breathable gas systems supplied by a source of pure oxygen. Thus, because it is physiologically unacceptable to breathe air that is over-enriched with oxygen in relation to the ambient pressure, i.e. cabin pressure, to which the aircrew is being subjected, the substantially pure oxygen product gas is diluted with air.
However there are proposals for systems in which the sieve beds are induced to deliver product gas having a variable oxygen concentration adapted to the aircrew breathing requirements and in one such system disclosed in GB-A-2,029,257 (Linde), this is achieved by spilling varying amounts of product gas from the delivery line by means of a valve so that the rate of flow of air through the beds is increased, and the amount of nitrogen adsorbed per unit volume of air is reduced, as required to give a product gas of desired oxygen content.
Control means for a spill valve in this system generally comprises means sensing the concentration of oxygen in the product gas prior to its entry into the breathing mask of an aircrew member, means sensing the pressure within the aircraft cabin, and means comparing the sensed oxygen concentration with cabin pressure and translating the result into a spill valve control signal.
Other disclosures of aircraft molecular sieve type oxygen enrichment of air systems for supplying oxygen enriched air as breathable gas for aircrew are to be found in U.S. Pat. No. 3,922,149 and U.K. Patent Application No. 2,013,101A.
Two principal factors are concerned in any solution to the problem of regulating oxygen concentration in oxygen enriched air supplied by a molecular sieve type gas separation system for breathing by aircrew in an aircraft application. One factor is the dependence of the system performance on the ratio of the sieve bed charge pressure to the sieve bed vent pressure, the charge pressure being dependent upon supply gas pressure and the vent pressure on the pressure of the environment to which the bed is vented. The other factor is the physiological requirements of the aircrew which relates the partial pressure of oxygen to ambient pressure (i.e. the concentration of oxygen in the breathable gas must be appropriately related to cabin pressure).